The present invention relates to a method of forming a planar optical waveguide, to a waveguide formed by such a method and to a method for tuning or xe2x80x9ctrimmingxe2x80x9d the waveguide.
It is known to manufacture silica-based optical waveguides at a temperature above the melting point of the waveguide material by deposition techniques such as flame hydrolysis of a suitable precursor powder. It has been proposed, for example in U.S. Pat. No. 5,117,470 to Inoue et al., to thermally process a waveguide which is initially formed at a high temperature (about 1200xc2x0 C.), so as to adjust the effective refractive index. The thermal processing involves heating the waveguide to a temperature which is below its formation temperature and melting point, followed by rapidly cooling the waveguide to room temperature. The process of post-deposition adjustment of the effective refractive index of a waveguide is sometimes referred to as xe2x80x9ctrimmingxe2x80x9d.
One disadvantage of this prior art trimming process is that the initial formation temperature is quite high. Where the waveguide is integrated with other components such as active optical device structures or electronic elements, those structures typically cannot withstand temperatures in excess of 1000xc2x0 C. A further disadvantage is that the process described in U.S. Pat. No. 5,117,470, which is dependent on a rapid cooling after the heating step, is only effective in regions of the waveguide which contain particular dopants, such as in the cladding layers of the waveguide.
It is thus an object of the present invention to provide a method of forming a waveguide that at least partially ameliorates a disadvantage of the prior art.
In a first aspect, the invention resides in a method of forming a planar waveguide comprising the steps of: forming a silica-based waveguide at a first temperature which is below a melting temperature of material from which the waveguide is formed; and annealing a region of the waveguide at a second temperature which is greater than the formation temperature and less than a melting temperature of material from which the waveguide is formed, so as to alter an effective refractive index of the region.
Preferably the step of annealing is preceded by the step of forming a thin film heater over the region of the waveguide, the heater being capable of heating the region to the second temperature.
Preferably the step of annealing is preceded by the step of analysing the formed waveguide to determine the refractive index profile of the waveguide.
In a second aspect, the invention resides in a method of forming a planar waveguide comprising the steps of selecting a first temperature for forming a silica based waveguide which is below a melting temperature of material from which the waveguide is to be formed; forming said silica based waveguide at said first temperature from said material such that said waveguide is trimmable by a post formation annealing process on a region of said waveguide at a temperature between said first temperature and said melting temperature so as to alter the effective refractive index of the region.
Preferably the first temperature is selected so as to allow the refractive index of the region to be altered within an optimal range. It is thus preferred that the waveguide is formed at a temperature below 600xc2x0 C., more preferably below 500xc2x0 C. and still more preferably below 400xc2x0 C.
Preferably the waveguide comprises a core formed between a buffer layer and a cladding layer, and at least one of the core, the buffer layer and the cladding layer is deposited by plasma-enhanced chemical vapour deposition (PECVD).
Preferably the method further comprises the step of annealing said region of the waveguide at said second temperature which is greater than the selected formation temperature and less than the melting temperature of material from which the waveguide is formed, so as to alter the effective refractive index of the region.
The invention further resides in a planar waveguide formed by any of the above-described methods. Furthermore, the invention resides in an optical device incorporating a waveguide formed according to the methods of the invention. An optical device may be, without limitation, any one of a group comprising an arrayed-waveguide grating (AWG), a Mach-Zehnder interferometer, a directional coupler, a polarization beam splitter, an Nxc3x97M optical switch matrix, an optical modulator, an optical attenuator, a variable optical attenuator, an add/drop multiplexer and a variable add/drop multiplexer.